Triple-bonded boron opens new chemical world

In a vacuum-sealed flask on a lab bench in Germany sits an emerald-green crystal that will cause some jaws to drop. The crystal is the first stable compound containing a triple chemical bond between two boron atoms, a feat that had previously been limited to only two other non-metal elements – carbon and nitrogen.

"Carbon easily forms triple bonds, nitrogen easily forms triple bonds," says Holger Braunschweig of the Julius Maximilian University of Würzburg in Germany, who led the team of researchers that made the compound. "You could expect something similar for boron, but it hasn't been done before. Certainly this will go into the textbooks."

Boron occupies a special spot in the periodic table. Each atom has three outer electrons, the minimum number needed to form a triple bond. But the element is flanked on all sides of the table by atoms that take very different bonding strategies.

On one side are metals like beryllium, which can give away all their outermost electrons to form ionic bonds. On the other are so-called main group elements like carbon and nitrogen, which prefer sharing electrons in covalent bonds.

Carbon and nitrogen in particular can share up to three of their outer electrons with another atom of the same kind, forming triple bonds. These are stronger than single or double bonds and so result in some of the most stable molecules known.

Aloof atom

Boron is more aloof than these neighbours, though. So much so that in its stable solid state at room temperature, it barely even reacts with boiling acid. "The chemistry of boron truly is different from the chemistry of all other elements," says Braunschweig, who built his career coaxing boron into strange bonds.

In 2002, Mingfei Zhou and colleagues at Fudan University in Shanghai, China, managed to make a boron triple bond – but only by vaporising the boron with a laser at 8 degrees above absolute zero. At room temperature, it would last less than a second.

Though chemists ran quantum-mechanical calculations to predict what the properties of compounds with boron triple bonds would be if you could make them stable, it wasn't clear that this would ever be achieved.

"I was sceptical that anyone could make it as a stable molecule," says Gernot Frenking of the Philipp University of Marburg in Germany. "Braunschweig found a way."

Slot filler

It turns out that to coax boron atoms to form triple bonds at room temperature, Braunschweig and colleagues had to make sure all the spaces where it could hold outer electrons were occupied.

Boron has four outer slots that can hold up to two electrons each – but in atomic boron, one of these slots is completely empty and the other three are only half full, with one electron apiece.

First the team filled up the completely empty slot by bonding each boron atom with a carbon-and-nitrogen-containing molecule called an N-heterocyclic carbene, which donated two electrons. Boron atoms bound to the molecule then completed the filling of their slots by pairing up and pooling their original three electrons. Each boron atom then has a full outer suite of eight electrons (see diagram, above right).

As long as it is not exposed to air or moisture, with which it is known to react, the resulting green crystal with the rare triple bond should stay stable indefinitely, Braunschweig says.

Secret garden

It's too early to predict if or how the new compound might be useful. But boron's particular electronic structure has already earned it uses in colour LCDs and LEDs.

Braunschweig is now investigating whether the new boron compound can be made to react with small molecules like hydrogen or carbon monoxide to make further novel compounds. "We already did some preliminary reactions, and it turns out that the compound shows a rich chemistry. Some of that is without any precedent. I have high expectations."

A world of previously forbidden chemistry beckons. "They did not simply find a new flower or a new plant," Frenking says. "They opened the door to a new garden."

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